Are you asking how to change the dream plot/scene? In that case you could try on using a dream portal (Example: passing through a mirror)and expect it to take you where you want or you could try spinning and visualize the new scene.

If you are asking how to have a specific dream theme when you go to sleep, I cant help you much on that, what I do know is that during the day you are getting elements that are going to appear on your next dream. For example: Going to the beach the whole day could make you dream about it.